Ion implantation is a process used to dope impurity ions into a semiconductor substrate to obtain desired device characteristics. An ion beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. One or more ion species may be implanted at different energy and dose levels to obtain desired device structures. In addition, the beam dose (the amount of ions implanted per unit area in the substrate) and the beam current (the number of ions that pass through a given plane perpendicular to the direction of the velocity vector of the beam per unit time) can be manipulated to provide a desired doping profile in the substrate.
Vacuum conditions in components of an ion implanter are often created and maintained by a turbomolecular pump. Turbomolecular pumps are a type of kinetic vacuum pump, similar to that of a turbine. A turbomolecular pump is typically designed as including a multi-stage, turbine-like rotor with bladed disks rotates in a housing. The blades of a turbine or a compressor are referred to collectively as the blading. Interposed mirror-invertedly between the rotor disks are bladed stator disks having similar geometries. Such pumps operate on the principle that gas molecules can be given momentum in a desired direction by repeated collision with a moving solid surface. In a turbomolecular pump, a rapidly spinning turbine rotor hits gas molecules, forcing the molecules from the inlet of the pump towards the exhaust in order to create or maintain a vacuum. Gas captured by the upper turbine stages is impelled into the lower turbine stages and successively compressed to a backing pump pressure level. Due to the limited compression ratio defined as the ratio of the exit pressure to the inlet pressure, turbomolecular pumps often cannot discharge against atmospheric pressure. Instead, they may require that the turbomolecular pump exhaust be connected to a backing pump which produces an exhaust pressure low enough for the turbomolecular pump to work efficiently. Typically, the backing pump pressure seen by the turbomolecular exhaust is below 500 Pa and commonly about 10 Pa.
Turbomolecular pumps are deployed, for example, within a high voltage section of ion implanter 100, and are maintained at a high voltage potential (e.g., >180 kV) with respect to electrical ground. The backing pump is conventionally maintained at the same electrical potential as the turbomolecular pump, and therefore the backing pump is also at a high voltage potential with respect to electrical ground. Backing pumps have large motors and use three phase AC, power for operation. An ion implanter that employs a turbomolecular pump in the high voltage portion of the implanter requires that the backing pump be placed at the same potential as the turbomolecular pump. It is with respect to these and other considerations that the present improvements have been needed.